The United States of America is a major consuming market of narcotic drugs and a primary target of terrorist attacks. Drug dependence is a chronic, relapsing disorder that exacts an enormous cost on individuals, families, businesses, communities, and nations. Terrorist highjacking and bombings, especially those involving and targeting civilian airplanes, greatly jeopardize the lives of the public.
Drugs, explosives and weapons are transported illegally by various methods. Large objects such as trucks or cargo containers are checked at the southwest (U.S.-Mexico) border for large amounts of drugs or other goods of interest, while passenger""s luggage is screened for a small amount of explosives or drugs (100-300 grams) or for weapons at airports. There are a number of techniques currently used or proposed for defecting drugs, explosives and/or other contraband.
There have been many governmental efforts to develop detection technology for drugs, explosives and weapons led by the Office of National Drug Control Policy (ONDCP) and the Federal Aviation Administration (FAA). Commonly used methods for detecting drugs, explosives and/or weapons in parcels or cargo are described, below.
The oldest and perhaps the most reliable way of detecting hidden drugs and explosives is to check the suspected object manually. After the baggage or cargo container is opened and carefully searched, we can determine with a high level of confidence whether a weapon, an explosive device or an illegal drug is present. However, manual checking is slow, expensive and perhaps dangerous. In addition, manual inspection can violate societal norms and legal standards relating to privacy. Usually, only some randomly selected or highly suspected items are checked manually.
Use of a detector dog and its handler is another old, but still broadly used, technique for drugs and explosives detection. Canine olfactory systems are capable of reacting to the vapor of a number of compounds in drugs or explosives. When a dog is trained to detect a substance, it learns to discriminate the vapor of that substance from other odors in the environment by reacting to the compound(s) and earning positive reinforcement from the handler when it does so. A dog usually works for several hours a day but may not be willing to work at the required time. In addition, canine detection fails if no vapor is coming out of the package.
Drugs and explosives can also be detected by trace contaminants such as vapor or particulate residue. Air samples are collected, condensed, and then analyzed, very often with ion mobility spectrometer (IMS) detectors. Ways of collecting samples range from sniffing the vapors to wiping the surface, but they all depend on contamination. This detection method is used to screen people and/or boarding passes.
X-ray detection systems are the most commonly used for parcel and cargo inspection. X-ray transmission radiography or tomography is based on the attenuation of x-rays to image the electron density in the object under inspection. An experienced operator can identify different items by their density and shape distribution. X-ray systems are very effective in telling metal from organic material, but lack the ability to distinguish drugs or plastic explosives from ordinary goods. When low energy x-rays are used as the source, a second detector may be used to record the scattered x-rays. This generally helps to map low-Z material near the surface of the inspected object and is used to detect sheet explosives hidden in the parcel wall or other contraband beneath the wall of a cargo container. To reduce the cost and increase penetration, a radioactive gamma-ray source (137Cs or 60Co) is sometimes used to replace the x-ray machine.
The use of neutron resonance radiography (NRR) has been described for applications other than detection of items such as explosives, weapons and drugs. NRR is based on the fact that, for a given element, the absorption of a beam of neutrons varies with neutron energy. Neutron absorption for a given element will typically show a series of relatively narrow peaks (xe2x80x9cresonancesxe2x80x9d) as a function of energy. If an image is taken at an energy matching the resonance energy and another image is taken at an energy off the resonance energy for a particular element, the difference of the two images will provide an enhanced image of that element. Thus, with appropriate choices of energies, an image may be taken of a particular element (e.g., carbon) within the object.
Neutrons of appropriate energies may be produced using a particle accelerator to accelerate beams of charged particles, such as protons or deuterons, onto a solid or gas target. In the usual method of performing resonance radiography, the energy of the accelerator is varied so as to produce beams of neutrons with a narrow range of energies. Each element is imaged at two energies. Further, the target used is made relatively thin in order to produce neutrons with a narrow range of energies. Such thin targets are inefficient producers of neutrons and also limit the accelerator output due to heating in the target.
Methods and apparatus for mapping the elemental composition of objects, particularly parcel-size objects, by means of neutron resonance radiography (NRR) are further described, below. In methods of this disclosure, neutrons are produced in a relatively broad band of energies from an efficient thick target, and the angular dependence of the neutron energy can be used to scan multiple energies simultaneously and to measure several elements simultaneously. Further, the energy of the particle accelerator can be fixed; and thus, complications involved with changing accelerator energies are avoided. Compact fixed-energy accelerators may thus be deployed.
Unlike other resonance techniques, such as pulsed fast neutron transmission spectroscopy (PFNTS), the methods and apparatus disclosed herein do not require an accelerator that produces short pulses of neutrons; moreover, these same methods and apparatus can use a simple large imager as a detector. Complexity in both the accelerator and the detector is consequently avoided. In contrast, the PFNTS method determines neutron energy by measuring the velocity of the neutrons, and thus requires both a very short (on the order of nanoseconds) pulse of neutrons and a relatively long distance over which to measure their time of flight so as to determine energy, thereby increasing the space required for such a system.
In NRR methods further described below, two-dimensional elemental mapping of, for example, hydrogen, carbon, nitrogen, oxygen and the sum of other elements can be calculated using fast neutron radiographic images taken at different neutron energies chosen to cover the resonance features of one or more elements. A radiographic image provides the two-dimensional mapping of the sum of elemental contents weighted by their attenuation coefficients, and images taken at different neutron energies (at which the elements will have different attenuation coefficients) form a set of linear equations for each pixel in the image. These equations can then be solved to map individual elemental content. Contraband, such as explosives and drugs, can be identified by their characteristic elemental composition. Accordingly, pieces of passenger luggage can be imaged, in an airport, for example, with the methods taught herein to detect contraband such as explosive materials or drugs (e.g., cocaine) by their characteristic elemental compositions. Neutrons having different energies (e.g., 2-6 MeV) can be obtained at different angles from a deuteron-deuteron (DD) neutron source. A fixed-energy radio frequency quadrupole (RFQ) accelerator with a thick target can be used as the neutron source in NRR.